Fuel cell systems have already been devised or already introduced into commercial aircraft for accomplishing various tasks. Besides the generation of electricity, other tasks can be accomplished, such as inerting a fuel tank by means of the exhaust gases from a fuel cell system. In general, if moist air is used in order to inert a fuel tank the problem arises in that fuel, and kerosene in particular, is hygroscopic and in addition a population of bacteria can arise that can affect the sensors that determine the level of fuel in the tanks and cause them to malfunction and even to form ice crystals, which can damage the engine injector nozzles and the fuel filters when the aircraft is in flight or on the ground at temperatures below freezing point.
DE 10 2005 054 885 A1 and US 2007/0111060 A1 disclose a safety system for reducing the risk of explosions in a fuel tank, in which a protective gas generation device incorporates a fuel cell system with a fuel cell and produces a protective gas generated by the fuel cell during the operation of the fuel cell system.
According to the current state of the art, different systems and processes are known for the drying of air. In this way, for example, it would be possible to create adsorption through the use of hygroscopic media, for example silica gel. However, the ability of a hygroscopic medium to absorb water is not limitless, and it therefore needs to be either replaced or regenerated. In an aircraft in particular, any such replacement will result in pronounced weight problems, while the constant emptying and refilling adds significantly to the maintenance costs. At the same time, regeneration could be possible through an appropriate introduction of heat, for example by heated air. However, this would compromise the effectiveness of the fuel cell system, as a considerable energy expenditure would be required for thermal regeneration. If regeneration shall not be carried out, gas drying is only possible for a limited time. In general, in such processes, dew points—that is to say temperatures at which condensing and evaporating water are in a state of equilibrium—arise, which are in the region of double negative figures.
A further process for drying air involves water transfer by means of a selective membrane and the use of a partial pressure difference. Here, a membrane is used, which separates gas that has yet to be dried from gas that has already been dried, whereby the passage of water through the membrane takes place as a result of the partial pressure difference. As an alternative to the very dry gas, the static pressure can be increased on the side of the membrane on which the gas to be dried is located. This process is restricted by the partial pressure difference that is attainable in its drying capacity. Especially low dew points for a membrane pressure air dryer can only be achieved if a very high operating pressure is used in conjunction with the necessarily high compressor capacity.
A further third process is known from the state of the art for drying gases, which involves cooling the gas to below the dew point, which requires only a heat exchanger and a heat sink, or a cooling medium. In addition to the cooling, and for the final separation of liquid water and gaseous residual gas, a drip tray is usually required. However, this principle requires a really high cooling capacity, as liquid product water is present and the energy that is released at the phase transition has to be removed. The cold required to cool the gas can partly be retained in an attached recuperative heat exchanger. In principle, the attainable dew point is limited by the freezing point, as because of the icing arising within the heat exchanger through the current construction method, the gas channels may become blocked.